The present invention relates generally to technology for detecting an analyte. In various embodiments, the invention relates to devices for measuring pH, the potential of hydrogen, which is a measure of the acidity or alkalinity of a solution. The pH of a solution is determined by the concentration of dissolved hydrogen ions (H+) (also referred to as hydronium ions, H3O+) within the solution. As the concentration of dissolved hydrogen ions within the solution increases, the solution becomes more acidic. Conversely, the solution becomes more basic as the concentration of dissolved hydrogen ions within the solution decreases. The concentration of dissolved hydrogen ions within a solution has traditionally been measured with a glass electrode connected to an electronic meter that displays the pH reading. Traditionally the terms “probe” and “electrode” have been used interchangeably to describe a functional grouping of component electrodes. As used herein, the term “electrode” is used to refer to a specific electrode in a probe, i.e., such as a “working electrode”, a “reference electrode”, or a “counter electrode”, and “probe” refers to a functional grouping of electrodes sufficient to generate a signal that can be processed to generate a reading indicative of the concentration of an analyte of interest in a solution.
The traditional glass pH probe has a working electrode (WE) that is an ion-selective electrode made of a fragile, doped glass membrane sensitive to hydrogen ions. The pH-responsive glass membrane is the primary analyte sensing element in this type of probe and so is referred to as the “working” electrode. Hydrogen ions within the sample solution bind to the outside of the glass membrane, thereby causing a change in potential on the interior surface of the membrane. This change in potential is measured against the constant potential of a conventional reference electrode (RE), such as an electrode based on silver/silver chloride. The difference in potential is then correlated to a pH value by plotting the difference on a calibration curve. The calibration curve is created through a tedious, multistep process whereby the user plots changes in potential for various known buffer standards. Traditional pH meters are based on this principle.
The response of traditional glass working electrodes (and probes and meters containing them) to pH is unstable, and glass probes periodically require careful calibration involving tedious, time-consuming processes, multiple reagents, and a trained operator. The special properties and construction of the glass probes further require that the glass membrane be kept wet at all times. Thus, routine care of the glass probe requires cumbersome and costly storage, maintenance, and regular calibration performed by a trained operator to ensure proper working performance.
In addition to tedious maintenance and storage requirements, traditional glass probes are fragile, thereby limiting the fields of application of the glass probe. In particular, the fragile nature of the glass probe makes it unsuitable for use in food and beverage applications, as well as use in unattended, harsh, or hazardous environments. Accordingly, there is a need in the art for pH probes and meters (as well as other analyte probes and meters) that address and overcome the limitations of traditional pH probes and meters employing the glass probe. Voltammetry-based analyte sensing systems have been proposed as a replacement for the glass probe; however, those systems were costly and difficult to use when first developed (see Wrighton, U.S. Pat. No. 5,223,117).
Significant advances were made in both theory and research laboratory practice of voltammetry-based analyte sensing systems when researchers discovered that carbon could replace gold as the conductive substrate and, moreover, that, regardless of the substrate, mixtures of redox active materials could be used in voltammetric systems (see PCT Pub. Nos. 2005/066618 and 2005/085825). One particularly intriguing proposal by these researchers was that a mixture of “analyte-sensitive” redox active materials (ASMs) and “analyte-insensitive” redox active materials (AIMs) could be attached to a conductive substrate and effectively convert it into both a WE (signal generated by the ASM) and a reference electrode (RE) (signal generated by the AIM). No significant advances, however, in either theory or practice were made for some time after these initial proposals and research (see, e.g., PCT Pub. Nos. 2007/034131 and 2008/154409).
Another significant advance in the field occurred when scientists discovered that, in practice, no redox active material is completely “analyte-insensitive” and that practical application of voltammetric technology should focus on WEs without AIMs. These scientists also discovered, however, that, regardless of whether a redox active material was characterized as an ASM or AIM (collectively referred to herein as “redox active materials” or “RAMs”), it could be made truly analyte-insensitive by sequestration in an ionic medium or “constant chemical environment”. This discovery led to the analyte-insensitive electrode or AIE, which could not only be used as a replacement of the conventional RE in traditional pH measuring systems but could also be used with WEs based on voltammetry. See PCT Pub. No. 2010/104962. Soon after these discoveries, pH meters suitable for use on the laboratory bench-top and for important research and development applications were created. See PCT Pub. Nos. 2010/111531 and 2010/118156. More recent advances in ASM chemistry, electrode design, and fabrication technology have produced WEs and other components that collectively provide improved accuracy, minimal signal drift, and convenience of use such as wet-dry reversibility. See co-pending PCT application US2013/023029, incorporated herein by reference.
There remains a need in the art for electrodes, probes, pH meters, and other analyte sensing devices based on voltammetry that provide precise measurements over extended lifetimes and that can be used under a wider variety of conditions by relatively unskilled workers. In addition, conventional pH electrodes are limited to sizes and shapes required by glass fabrication technology. Thus the great majority of pH electrodes in use are straight, rigid rods of limited length. An alternative pH sensor that can be packaged in flexible, semi-rigid, or user-configurable form factors that also incorporate benefits from the abovementioned recent advances would enable many new applications. The present invention meets these needs.